A charged particle beam apparatus represented by a scanning electron microscope scans a sample with a thinly converged charged particle beam, and forms a sample image on the basis of a signal emitted from the sample. Such charged particle beam apparatuses have an increasingly higher resolution, and increasingly higher ratios of magnification for observation are required.
As a result of the increase in the observation magnification ratios, the issue of sample image reliability has arisen, as described below. When the sample is scanned with the charged particle beam, depending on the material of the sample, the sample may be shrunk or a carbon-based deposit may become attached to the surface of the sample (contamination) due to dissociation of hydrocarbon-based residual gas in the apparatus and the sample caused by the energy of the beam, such that the observed image of the sample may become different from its shape prior to observation.
Further, when the sample is scanned with the charged particle beam, a charging bias may be produced in the observed area, resulting in a sample image different from the actual shape of the sample. It is known that the charging bias is dependent on the following various contributing factors.
a) Changes depending on the energy and current density of the primary electron beam, observation magnification ratio, and the secondary electron/backscattered electron yield of the sample.
b) Temporal changes depending on electron movement or diffusion, annihilation by electron hole recombination and the like.
c) Changes depending on scan method.
As a result of the above phenomena, observation of the sample causes sample image distortion and a significant decrease in sample image reliability. Various methods for decreasing the sample image distortion have been considered.
Patent Literature 1 describes a technique for suppressing the sample shrinkage or contamination due to electron beam irradiation. In this technique, the scan interval of the electron beam is increased and the length of the scan area in the Y-direction is increased compared with the length in the X-direction such that the scan area is rectangular, and the amount of electron beam irradiation per unit area of the sample is decreased.
Patent Literature 2 describes a technique for suppressing sample image distortion due to the influence of localized charging of the sample by electron beam irradiation. The technique involves setting the order of raster scan in the vertical direction of the screen such that the center of arbitrary two lines that have been previously scanned is scanned at all times.
Patent Literature 3 describes a technique for suppressing sample image distortion due to changes in charging potential in sample areas with different charge characteristics when the sample is scanned with electron beam. The technique involves measuring the charging in each area of the sample and grasping electric characteristics so that setting conditions for charging or neutralizing means can be optimized.